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Bug 739133 - (CVE-2017-17785) Heap overflow while parsing FLI files. 

It is possible to trigger a heap overflow while parsing FLI files. The
RLE decoder is vulnerable to out of boundary writes due to lack of
boundary checks.

The variable "framebuf" points to a memory area which was allocated
with fli_header->width * fli_header->height bytes. The RLE decoder
therefore must never write beyond that limit.

If an illegal frame is detected, the parser won't stop, which means
that the next valid sequence is properly parsed again. This should
allow GIMP to parse FLI files as good as possible even if they are
broken by an attacker or by accident.

While at it, I changed the variable xc to be of type size_t, because
the multiplication of width and height could overflow a 16 bit type.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
This commit is contained in:
Tobias Stoeckmann 2017-10-29 15:19:41 +01:00 committed by Jehan
parent fff57f84a0
commit edb251a7ef
1 changed files with 35 additions and 15 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

50
plug-ins/file-fli/fli.c
View File

@ -25,6 +25,8 @@
#include "config.h"
#include <glib/gstdio.h>
#include <string.h>
#include <stdio.h>
@ -461,23 +463,27 @@ void fli_read_brun(FILE *f, s_fli_header *fli_header, unsigned char *framebuf)
unsigned short yc;
unsigned char *pos;
for (yc=0; yc < fli_header->height; yc++) {
unsigned short xc, pc, pcnt;
unsigned short pc, pcnt;
size_t n, xc;
pc=fli_read_char(f);
xc=0;
pos=framebuf+(fli_header->width * yc);
n=(size_t)fli_header->width * (fli_header->height-yc);
for (pcnt=pc; pcnt>0; pcnt--) {
unsigned short ps;
ps=fli_read_char(f);
if (ps & 0x80) {
unsigned short len;
for (len=-(signed char)ps; len>0; len--) {
for (len=-(signed char)ps; len>0 && xc<n; len--) {
pos[xc++]=fli_read_char(f);
}
} else {
unsigned char val;
size_t len;
len=MIN(n-xc,ps);
val=fli_read_char(f);
memset(&(pos[xc]), val, ps);
xc+=ps;
memset(&(pos[xc]), val, len);
xc+=len;
}
}
}
@ -565,25 +571,34 @@ void fli_read_lc(FILE *f, s_fli_header *fli_header, unsigned char *old_framebuf,
memcpy(framebuf, old_framebuf, fli_header->width * fli_header->height);
firstline = fli_read_short(f);
numline = fli_read_short(f);
if (numline > fli_header->height || fli_header->height-numline < firstline)
return;
for (yc=0; yc < numline; yc++) {
unsigned short xc, pc, pcnt;
unsigned short pc, pcnt;
size_t n, xc;
pc=fli_read_char(f);
xc=0;
pos=framebuf+(fli_header->width * (firstline+yc));
n=(size_t)fli_header->width * (fli_header->height-firstline-yc);
for (pcnt=pc; pcnt>0; pcnt--) {
unsigned short ps,skip;
skip=fli_read_char(f);
ps=fli_read_char(f);
xc+=skip;
xc+=MIN(n-xc,skip);
if (ps & 0x80) {
unsigned char val;
size_t len;
ps=-(signed char)ps;
val=fli_read_char(f);
memset(&(pos[xc]), val, ps);
xc+=ps;
len=MIN(n-xc,ps);
memset(&(pos[xc]), val, len);
xc+=len;
} else {
fread(&(pos[xc]), ps, 1, f);
xc+=ps;
size_t len;
len=MIN(n-xc,ps);
fread(&(pos[xc]), len, 1, f);
xc+=len;
}
}
}
@ -691,7 +706,8 @@ void fli_read_lc_2(FILE *f, s_fli_header *fli_header, unsigned char *old_framebu
yc=0;
numline = fli_read_short(f);
for (lc=0; lc < numline; lc++) {
unsigned short xc, pc, pcnt, lpf, lpn;
unsigned short pc, pcnt, lpf, lpn;
size_t n, xc;
pc=fli_read_short(f);
lpf=0; lpn=0;
while (pc & 0x8000) {
@ -702,26 +718,30 @@ void fli_read_lc_2(FILE *f, s_fli_header *fli_header, unsigned char *old_framebu
}
pc=fli_read_short(f);
}
yc=MIN(yc, fli_header->height);
xc=0;
pos=framebuf+(fli_header->width * yc);
n=(size_t)fli_header->width * (fli_header->height-yc);
for (pcnt=pc; pcnt>0; pcnt--) {
unsigned short ps,skip;
skip=fli_read_char(f);
ps=fli_read_char(f);
xc+=skip;
xc+=MIN(n-xc,skip);
if (ps & 0x80) {
unsigned char v1,v2;
ps=-(signed char)ps;
v1=fli_read_char(f);
v2=fli_read_char(f);
while (ps>0) {
while (ps>0 && xc+1<n) {
pos[xc++]=v1;
pos[xc++]=v2;
ps--;
}
} else {
fread(&(pos[xc]), ps, 2, f);
xc+=ps << 1;
size_t len;
len=MIN((n-xc)/2,ps);
fread(&(pos[xc]), len, 2, f);
xc+=len << 1;
}
}
if (lpf) pos[xc]=lpn;